Systems and methods for dual-band modulation and injection-locking for coherent PON

ABSTRACT

An optical communication network includes a downstream optical transceiver. The downstream optical transceiver includes at least one coherent optical transmitter configured to transmit a downstream coherent dual-band optical signal having a left-side band portion, a right-side band portion, and a central optical carrier disposed within a guard band between the left-side band portion and the right-side band portion. The network further includes an optical transport medium configured to carry the downstream coherent dual-band optical signal from the downstream optical transceiver. The network further includes at least one modem device operably coupled to the optical transport medium and configured to receive the downstream coherent dual-band optical signal from the optical transport medium. The at least one modem device includes a downstream coherent optical receiver, and a first slave laser injection locked to a frequency of the central optical carrier.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/453,836, filed Jun. 26, 2019. U.S. patent application Ser. No.16/453,836 claims the benefit of and priority to U.S. Provisional PatentApplication Ser. No. 62/689,954, filed Jun. 26, 2018. Both of theseprior applications are incorporated herein by reference in theirentireties.

BACKGROUND

The field of the disclosure relates generally to fiber communicationnetworks, and more particularly, to coherent optical networks utilizinginjection locking.

Telecommunications networks include an access network through which enduser subscribers connect to a service provider. Bandwidth requirementsfor delivering high-speed data and video services through the accessnetwork are rapidly increasing to meet growing consumer demands. Atpresent, data delivery over the access network is growing bygigabits/second (Gb/s) for residential subscribers, and by multi-Gb/sfor business subscribers. Many conventional access networks are nowbased on passive optical network (PON) access technologies, which havebecome the dominant system architecture to meet the growing highcapacity demand from end users in optical transport networks (OTNs).Some conventional PON architectures include Gigabit PON (GPON) andEthernet PON (EPON) architectures, and time and wavelength divisionmultiplexing (TWDM and WDM) have been recently been standardized in theconventional PON. A growing need therefore exists to develophigher/faster data rates per-subscriber to meet future bandwidth demand,but while also minimizing the capital and operational expendituresnecessary to deliver higher capacity and performance access networks.

Coherent technology has been proposed as one solution to increase bothreceiver sensitivity and overall capacity for WDM-PON optical accessnetworks, in both brown and green field deployments. In the downlink(DL) of conventional PONs, the complexity of the architecture limits thetransceiver in an optical line terminal (OLT) at the headend, centraloffice, and/or hub, but less so than the limits on the receiver in anoptical network unit (ONU), since the cost of the OLT transceiver, whichsends and receives data to and from multiple ONUs, is shared by all endusers supported in the respective network. In contrast, the cost of eachONU is born solely by the respective end user. Accordingly, lower costsand lower complexities will more significantly impact the ONU than theOLT. For this reason, the complexity and high cost of conventionalcoherent optical transceivers has been confined to point-to-point (P2P)implementations, which typically involve high-capacity business users,but prohibitive in point-to-multipoint (P2MP) PON applications, whichcommonly involve residential home subscribers. That is, despite thesignificant advantages offered by digital coherent technology, thecomplexity and high cost of conventional coherent transceivers has notbeen economically feasible for individual ONUs at the home location ofeach subscriber end-user.

P2P and P2MP applications differ in that the P2P connection provides alink between one transmitter and one receiver, whereas a P2MPapplication provides a link between one transmitter and multiplereceivers. Accordingly, in the coherent paradigm, only two coherenttransceivers are necessary in a P2P link, whereas the number of coherenttransceivers needed in the P2MP link, i.e., one coherent transceiver foreach ONU, may greater than 500 for each OLT. Thus the laser source is ofcritical importance for the successful realization of coherent OTNs, andone type of laser may not simply be substituted for another laser typewithout significantly affecting the network performance.

Recent advancements in digital coherent optical technologies inlong-haul transmission systems bring significant capacity improvementsOTNs. To date, with the trends of increased integration density andfabrication capacity of silicon photonic chips, coherent technology isenabled for further penetration into the access networks/OTNs, whichprofoundly impacts future network design. In the future-proof coherentaccess network, one major concern is the cost of the coherenttransceivers. A major contributor to the high cost of coherent opticalsystems arises from the narrow linewidth laser used to provide the lightsource for both the transmitter and the local oscillator (LO).

Some conventional coherent transceivers use an external cavity laser(ECL). From the performance perspective, ECLs have demonstrated superiorperformance capabilities for coherent systems, sufficient for presentlong haul and metro distance sensitivity requirements. However, withinthe access environment, ECLs are considered prohibitively expensive ifdeployed at each ONU at an end user's home location. In contrast,Fabry-Perot laser diodes (FPLD) and weak-resonant-cavity laser diodes(WRC-FPLD) are more commonly used in ONU transmitters, since such lasersare considerably less expensive than the costly externally tunablelasers (e.g., ECLs, distributed feedback (DFB)/distributed Braggreflector (DBR)). However, use of these relatively lower-cost, simpler(e.g., FP) lasers is limited by transmission bandwidth and capacity,particularly in direct-detection systems, and is not applicable forcoherent systems in the conventional use form.

To address these prohibitive laser cost concerns, improved systems andmethods for coherent optics with injection locking (COIL) are taught inU.S. Pat. No. 9,912,409, issued Mar. 6, 2018, and in co-pending U.S.patent application Ser. No. 16/408,285, filed May 9, 2019, thedisclosures of which are incorporated by reference herein. Theseimproved COIL systems and methods enables the use of relativelyinexpensive lasers (e.g., FPLDs) at the ONU/receiver site as a slavelaser, injection-locked by a narrow linewidth laser provided in thedownstream direction from the corresponding OLT/transmitter site.However, several challenges still remain with respect to theimplementation of these improved COIL techniques. For example, when thecoherent optical system relies on reuse of the modulated downstreamlight as the master laser, the signal quality of the upstreamtransmission might be significantly degraded by an incomplete erasingeffect, and the linewidth of the modulated light may be significantlybroadened, which may introduce strong phase noise in coherent systems.Additionally, to guarantee the signal quality after injection-locking,an unmodulated pure optical tone is still necessary for the master lightsource, which adds to the cost if an extra laser is deployed at the OLTto serve as the master laser. Accordingly, it is desirable to implementa coherent optical network system utilizing COIL, and which overcomesthe known challenges of these improved COIL techniques.

BRIEF SUMMARY

In an embodiment, an optical communication network includes a downstreamoptical transceiver. The downstream optical transceiver includes atleast one coherent optical transmitter configured to transmit adownstream coherent dual-band optical signal having a left-side bandportion, a right-side band portion, and a central optical carrierdisposed within a guard band between the left-side band portion and theright-side band portion. The network further includes an opticaltransport medium configured to carry the downstream coherent dual-bandoptical signal from the downstream optical transceiver. The networkfurther includes at least one modem device operably coupled to theoptical transport medium and configured to receive the downstreamcoherent dual-band optical signal from the optical transport medium. Theat least one modem device includes a downstream coherent opticalreceiver, and a first slave laser injection locked to a frequency of thecentral optical carrier.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic illustration of an exemplary coherent passiveoptical network communication system, in accordance with an embodimentof the present disclosure.

FIGS. 2A-C are graphical illustrations of optical spectra at differentrespective locations within the coherent passive optical networkcommunication system depicted in FIG. 1.

FIG. 3 is a graphical illustration depicting an exemplary signalgeneration process for a dual-band coherent optical signal.

FIG. 4 is a schematic illustration depicting an exemplary modulator formodulating the dual-band signal generated by the signal generationprocess depicted in FIG. 3.

FIG. 5 is a schematic illustration depicting an exemplary signalrecovery process performed by the downstream coherent optical receiverdepicted in FIG. 1.

FIGS. 6A-D are graphical illustrations depicting simulated resultsaccording to the exemplary embodiments described herein.

Unless otherwise indicated, the drawings provided herein are meant toillustrate features of embodiments of this disclosure. These featuresare believed to be applicable in a wide variety of systems including oneor more embodiments of this disclosure. As such, the drawings are notmeant to include all conventional features known by those of ordinaryskill in the art to be required for the practice of the embodimentsdisclosed herein.

DETAILED DESCRIPTION

In the following specification and the claims, reference will be made toa number of terms, which shall be defined to have the followingmeanings.

The singular forms “a,” “an,” and “the” include plural references unlessthe context clearly dictates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where the event occurs and instances where it does not.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” “approximately,” and “substantially,” are notto be limited to the precise value specified. In at least someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value. Here and throughout thespecification and claims, range limitations may be combined and/orinterchanged; such ranges are identified and include all the sub-rangescontained therein unless context or language indicates otherwise.

As used herein, the terms “processor” and “computer” and related terms,e.g., “processing device”, “computing device”, and “controller” are notlimited to just those integrated circuits referred to in the art as acomputer, but broadly refers to a microcontroller, a microcomputer, aprogrammable logic controller (PLC), an application specific integratedcircuit (ASIC), and other programmable circuits, and these terms areused interchangeably herein. In the embodiments described herein, memorymay include, but is not limited to, a computer-readable medium, such asa random access memory (RAM), and a computer-readable non-volatilemedium, such as flash memory. Alternatively, a floppy disk, a compactdisc-read only memory (CD-ROM), a magneto-optical disk (MOD), and/or adigital versatile disc (DVD) may also be used. Also, in the embodimentsdescribed herein, additional input channels may be, but are not limitedto, computer peripherals associated with an operator interface such as amouse and a keyboard. Alternatively, other computer peripherals may alsobe used that may include, for example, but not be limited to, a scanner.Furthermore, in the exemplary embodiment, additional output channels mayinclude, but not be limited to, an operator interface monitor.

Further, as used herein, the terms “software” and “firmware” areinterchangeable, and include computer program storage in memory forexecution by personal computers, workstations, clients, and servers.

As used herein, the term “non-transitory computer-readable media” isintended to be representative of any tangible computer-based deviceimplemented in any method or technology for short-term and long-termstorage of information, such as, computer-readable instructions, datastructures, program modules and sub-modules, or other data in anydevice. Therefore, the methods described herein may be encoded asexecutable instructions embodied in a tangible, non-transitory, computerreadable medium, including, without limitation, a storage device and amemory device. Such instructions, when executed by a processor, causethe processor to perform at least a portion of the methods describedherein. Moreover, as used herein, the term “non-transitorycomputer-readable media” includes all tangible, computer-readable media,including, without limitation, non-transitory computer storage devices,including, without limitation, volatile and nonvolatile media, andremovable and non-removable media such as a firmware, physical andvirtual storage, CD-ROMs, DVDs, and any other digital source such as anetwork or the Internet, as well as yet to be developed digital means,with the sole exception being a transitory, propagating signal.

Furthermore, as used herein, the term “real-time” refers to at least oneof the time of occurrence of the associated events, the time ofmeasurement and collection of predetermined data, the time for acomputing device (e.g., a processor) to process the data, and the timeof a system response to the events and the environment. In theembodiments described herein, these activities and events occursubstantially instantaneously.

As used herein, “modem termination system” (MTS) refers to a terminationunit including one or more of an Optical Network Terminal (ONT), anoptical line termination (OLT), a network termination unit, a satellitetermination unit, a cable modem termination system (CMTS), and/or othertermination systems which may be individually or collectively referredto as an MTS.

As used herein, “modem” refers to a modem device, including one or morea cable modem (CM), a satellite modem, an optical network unit (ONU), aDSL unit, etc., which may be individually or collectively referred to asmodems.

As described herein, a “PON” generally refers to a passive opticalnetwork or system having components labeled according to known namingconventions of similar elements that are used in conventional PONsystems. For example, an OLT may be implemented at an aggregation point,such as a headend/hub, and multiple ONUs may be disposed and operable ata plurality of end user, customer premises, or subscriber locations.Accordingly, an “uplink transmission” refers to an upstream transmissionfrom an end user to a headend/hub, and a “downlink transmission” refersto a downstream transmission from a headend/hub to the end user, whichmay be presumed to be generally broadcasting continuously (unless in apower saving mode, or the like).

The systems and methods described herein address and solve thechallenges, described above, presented with implementation of the recentinnovative developments in COIL. In an embodiment, a dual-band coherentoptical signal is utilized in the downlink, including a “pure” opticaltone at the center of the dual-band coherent optical signal. The pureoptical tone, which inherits the narrow linewidth properties of the highquality source laser/master laser may then be filtered out at the ONUsite to injection-lock a FPLD at the ONU.

System Configuration

FIG. 1 is a schematic illustration of an exemplary coherent PONcommunication system 100. In an exemplary embodiment, system 100 is aPON, and includes an OLT 102 (e.g., within or in communication with amodem termination system of an optical hub, headend, or central office(not separately illustrated)) and an ONU 104 (e.g., within or proximatea residential or business end-user modem, downstream termination unit,customer device, customer premises equipment, etc.) in operablecommunication with OLT 102 over an optical transport medium 106 (e.g., asingle mode fiber (SMF)). In the exemplary embodiment, system 100represents a coherent dense wavelength division multiplexing (DWDM) PONarchitecture. OLT 102 includes a downstream coherent optical transmitter108, an upstream coherent optical receiver 110, and a first opticalcirculator 112. ONU 104 includes a downstream coherent optical receiver114, an upstream modulator 116, a second optical circulator 118, anoptical filter 120, and a receiver laser 122 and a modulator laser 124(e.g., FPLDs).

System 100 may include several more additional components that are notillustrated in FIG. 1 for ease of explanation of the presentembodiments. Structurally, system 100 may be similar to convention PONarchitectures, that is, the basic hardware configuration of the opticalreceivers at the hub site (e.g., downstream coherent optical transmitter108 and upstream coherent optical receiver 110) need not change, therebyenabling greater ease of implementation into existing optical systems.In contrast to the conventional PON implementations, which employ adownstream single-carrier signal with quadrature amplitude modulation(QAM), as described further below with respect to FIGS. 2A-C, system 100applies a dual-band coherent optical signal in the downstream.

According to the advantageous configuration illustrated in FIG. 1, alllight sources at the user site (e.g., ONU 104) are based on inexpensiveand simple FPLDs as slave lasers, and no ECL (or similar) is required.This implementation of slave lasers in a COIL subsystem greatly reducesthe cost of the user site, as described in co-pending U.S. patentapplication Ser. No. 16/408,285. In the COIL subsystem, since the slavelasers clone the properties of the master laser having reduced narrowlinewidth, the performance of the coherent transceivers will be improvedin comparison with conventional schemes that re-modulate over thedownstream signal.

FIGS. 2A-C are graphical illustrations of optical spectra 200, 202, 204at different respective locations within coherent PON communicationsystem 100, FIG. 1. In the exemplary embodiment optical spectra 200,202, 204 represent the optical spectrum of the signal seen at points A,B, and C marked in system 100. More specifically, optical spectrum 200is taken at point A, between downstream coherent optical transmitter 108and first optical circulator 112, FIG. 1. Optical spectrum 202represents the response taken at point B of optical filter 120, FIG. 1,before reception by receiver laser 122. Optical spectrum 204 is taken atpoint C, between upstream modulator 116 and second optical circulator118, FIG. 1.

As illustrated in FIG. 2A, optical spectrum 200 is a dual-band coherentoptical signal including a guard band 206, which is centered with a pureoptical carrier 208 between a left-side band (LSB) 210 and right-sideband (RSB) 212. The generation and recovery/demodulation of thedual-band coherent optical signal 200 is described further below withrespect to FIGS. 3 and 5, respectively.

In further exemplary operation of system 100, as further considered withrespect to FIGS. 2A-C, dual-band coherent optical signal 200 passesthrough first optical circulator 112 and travels from OLT 102downstream, along optical transport medium 106, to ONU 104. At the enduser site (e.g., ONU 104), dual-band coherent optical signal 200 passesthrough second optical circulator 118 and is then separated into twoparts along two separate paths 126(1), 126(2), respectively. The firstpart (i.e., path 126(1)) feeds directly into downstream coherent opticalreceiver 114 as the light signal, which substantially includes all ofdual-band coherent optical signal 200. The second part (i.e., path126(2)) feeds into optical filter 120. In an exemplary embodiment, andas illustrated in FIG. 2B, optical filter 120 applies a bandpass filter(BPF) 214 to dual-band coherent optical signal 200, which extractscentral optical carrier 208′ from LSB 210 and RSB 212. Extracted centraloptical carrier 208′ may then be used to generate the LO light that willbeat with the signal light inside downstream coherent optical receiver114 for demodulation.

In the exemplary embodiment, BPF 214 is selected to have an opticalfilter that generally corresponds to a frequency range of guard band206, and optical filter 120 may, for example, be a narrowband Bragggrating or a wave shaper. The filtered optical carrier at point B ofFIG. 1 (e.g., optical spectrum 202) be then be used to injection-lockthe FPLD of receiver laser 122. Accordingly, utilizing thereamplification and erasing effect from the FPLD of receiver laser 122,the power of the optical carrier (e.g., extracted central opticalcarrier 208′) will be boosted to act as the LO.

In some embodiments, some part of the light signal, afterinjection-locking, is used to feed upstream (or uplink (UL)) modulator116, which is configured to convert an electrical signal into theoptical domain, and then output the UL signal of optical spectrum 204,as illustrated in FIG. 2C, which may then be transmitted back to OLT 102over optical transport medium 106, after passing through second opticalcirculator 118. Depending on the polarization sensitivity of the FPLD ofreceiver laser 122 or modulator laser 124, a polarization controller 128may optionally included along the respective pathway between opticalfilter 120 and one or both of the injection-locked slave lasers 122,124. Optical injection locking (e.g. COIL) as described herein thusimproves upon the performance of more expensive multi-longitudinal slavelaser sources in terms of spectral bandwidth and noise properties.

Generation and Recovering of the Dual-Band Coherent Signal

FIG. 3 is a graphical illustration depicting an exemplary signalgeneration process 300 for a dual-band coherent optical signal. In anembodiment, signal generation process 300 may be employed to generatedual-band coherent optical signal 200, FIG. 2A. In the exemplaryembodiment, signal generation process 300 includes a first step 302 forproducing a pair of baseband coherent signals 304, a second step 306 forupconverting baseband coherent signals 304 into a pair of intermediatefrequency (IF) signals 308, respectively, and a third step 310 forcombining IF signals 308 into a double-side-band complex-valued signal312.

In an exemplary embodiment of production step 302, process 300 producesthe pair of baseband coherent signals 304 is in-phase (I) and quadrature(Q) coherent signals, or I/Q signals S₁ (i.e., baseband coherent signal304(1)) and S₂ (i.e., baseband coherent signal 304(2)), modulated by aQAM format. Baseband coherent signals 304(1), 304(2) may then bemathematically represented as:S ₁ =I ₁ +jQ ₁,  (Eq. 1)S ₂ =I ₂ +jQ ₂.  (Eq. 2)

In upconversion step 306, I/Q signals S₁ and S₂ are digitallyupconverted onto the intermediate frequencies, +ω_(IF) and −ω_(IF),respectively, to create IF signals 308(1), i.e., S_(IF1), and 308(2),i.e., S_(IF2), which may be mathematically represented as:S _(IF1) =S ₁×exp(jω _(IF) t)=(I ₁ +jQ ₁)exp(jω _(IF) t),  (Eq. 3)S _(IF2) =S ₂×exp(−jω _(IF) t)=(I ₂ +jQ ₂)exp(−jω _(IF) t).  (Eq. 4)

In combining step 310, the two IF signals S_(IF1), S_(IF2) (i.e.,308(1), 308(2), respectively) are combined together to formdouble-side-band complex-valued signal 312, which includes IF signal308(1)/S_(IF1) as an RSB portion 314 and IF signal 308(2)/S_(IF2) as anLSB portion 316. Double-side-band complex-valued signal 312, or S_(tot),may then be mathematically represented as:S _(tot) =S _(IF1) +S _(IF2)=[I ₁ +I ₂)cos(ω_(IF) t)−(Q ₁ −Q₂)sin(ω_(IF) t)]+j[(Q ₁ +Q ₂)cos(ω_(IF) t)+(I ₁ −I ₂)sin(ω_(IF)t)].  (Eq. 5)

As illustrated in FIG. 3, in addition to RSB portion 314 and LSB portion316, double-side-band complex-valued signal 312 further includes afrequency guard band Δω_(g) (e.g., guard band 206, FIG. 2A) between therespective portions 314, 316. After application of double-side-bandcomplex-valued signal 312 onto the modulator (e.g., upstream modulator116, FIG. 1), there will be a residual optical carrier 318 located atthe center of signal 312, and the guard band Δω_(g) thus guarantees thatresidual optical carrier 318 may be readily isolated from both RSBportion 314 and LSB portion 316. Accordingly, an optical bandpass filter(e.g., optical filter 120, FIG. 1) may be utilized to extract centraloptical carrier 318 for injection locking.

In one illustrative example, the baud rate of the baseband coherent QAMsignal at each side band (e.g., baseband signals 304(1), 304(2)) may beassumed to be Δf_(B). Thus, in comparison with a conventional coherentsingle-carrier signal having a baud rate of 2Δf_(B), the bandwidthefficiency of the dual-band coherent signal (e.g., dual-band signal 312)will be reduced by a factor of 2Δf_(B)/(2ω_(IF)+Δf_(B)). Accordingly, inthis example, a coherent PON system (e.g., system 100, FIG. 1) maysacrifice some amount of spectral efficiency in exchange for thecapability of easily filtering out the pure optical carrier (e.g.,optical carrier 318, optical carrier 208, FIG. 2A) forinjection-locking. This trade-off is valuable because (i) bandwidthrequirements are not as high in the optical access network as they arein long-haul transmission systems, and (ii) cost-effectiveness is, atpresent, the most important consideration at the user site or ONU,particularly within the P2MP paradigm. By reducing two narrow-bandwidthlasers (one for LO, and another fed to the modulator) at the ONU, thecost and complexity of each ONU is significantly reduced, but while theperformance of the particular ONU approximates that of an expensive,high quality laser source at the user site.

FIG. 4 is a schematic illustration depicting an exemplary modulator 400for modulating the dual-band signal generated by signal generationprocess 300, FIG. 3. In an exemplary embodiment, modulator 400 is an I/Qmodulator having a Mach-Zehnder modulator (MZM) architecture. Thisexemplary embodiment though, is described for illustrative purposes, andnot in a limiting sense. The person of ordinary skill in the art willunderstand, after reading and comprehending the present disclosure, thatother modulator structures and schemes may be employed without departingfrom the scope of the present application.

Referring back to FIG. 3, after generation of the dual-band signalS_(tot) (e.g., signal 312), the real and imaginary components (i.e.,real(S_(tot)) and imag(S_(tot))) thereof may be modulated onto anI-branch 402 and a Q-branch, respectively, of I/Q modulator 400. In thisexample, I/Q modulator 400 further includes a first MZM portion 406along I-branch 402 and a second MZM portion 408 along Q-branch 404.First and second MZM portions 406, 408 are both biased at a voltageV_(π), and a voltage of V_(π/2) is thus applied across I- and Q-branches402, 404.

FIG. 5 is a schematic illustration depicting an exemplary signalrecovery process 500 performed by downstream coherent optical receiver114, FIG. 1. In the exemplary embodiment, signal recovery process 500 isperformed to demodulate a modulated downstream dual-band coherentoptical signal (e.g., dual-band coherent optical signal 200, FIG. 2A).In the exemplary embodiment, signal recovery process 500 includes afirst step 502, in which an optical light signal (optical signal 200, inthis example) enters downstream coherent optical receiver 114, whichalso receives an LO signal 504, and optical signal 200 beats with LOsignal 504 to project onto two polarizations and two orthogonal phases.

In step 506, and output of downstream coherent optical receiver 114 isseparated into first and second signal portions 508(1), 508(2),respectively. In an exemplary embodiment, signal recovery process 500simultaneously processes first and second signal portions 508(1),508(2). In at least one embodiment, first and second signal portions508(1), 508(2) may be processed sequentially, at separate times, or in adifferent order.

In step 510, first signal portion 508(1) is digitally multiplied by afactor of exp(−jω_(IF)t). In an exemplary embodiment of step 510, secondsignal portion 508(2) is digitally multiplied by a factor ofexp(+jω_(IF)t) (e.g., simultaneously). In step 512, the RSB component(e.g., RSB 212, FIG. 2A) of first signal portion 508(1) is downconverted onto the baseband, and first filter 514(1) may be applied toeliminate the LSB component (e.g., LSB 210, FIG. 2A). In an exemplaryembodiment of step 512, after digital multiplication by theexp(+jω_(IF)t) factor, the LSB component of second signal portion 508(2)is similarly down converted and a second filter 514(2) may be applied toextract the RSB component thereof on the baseband. In some embodiments,one or both of filters 514 may be LPFs, depending on the particulardesign considerations of receiver 114 and the processing steps ofprocess 500. In other embodiments, the person of ordinary skill in theart will understand that particular BPFs and/or HPFs may be used forsimilar purposes in step 512 without departing from the scope herein.

Regardless, it may be noted that, in the exemplary embodiment, after thedown-conversion performed in step 512, two single-carrier basebandsignal components are generated having a QAM format. In step 516, bothof these generated single-carrier baseband signal components may then bedemodulated by a respective conventional coherent QAM demodulation DSP518.

FIGS. 6A-D are graphical illustrations depicting simulated results 600according to the exemplary embodiments described herein. In an exemplaryembodiment, results 600 represent selected experimental simulations ofrelevant optical spectra at various stages through will generationprocess 300, FIG. 3, and signal recovery process 500, described above.More particularly, FIG. 6A depicts a spectrum (i.e., power-vs-frequency)of a baseband signal 602 (e.g., baseband I/Q signal 304(1), S₁, FIG. 3).FIG. 6B depicts a spectrum (also power-vs-frequency) of a dual-bandsignal 604 (e.g., double-side-band complex-valued signal 312, S_(tot),FIG. 3). FIG. 6C depicts a spectrum (power-vs-frequency) of a basebandsignal 606 after down-conversion and filtering (e.g., step 512, FIG. 4).FIG. 6D graphically illustrates a recovered constellation 608 (64QAM, inthis example), which may, for example, be obtained after completion ofsignal recovery process 500 (e.g., step 516, FIG. 4).

In the simulation that was performed to generate results 600, the baudrate of the baseband single-carrier coherent signals S₁ and S₂ (e.g.,baseband signal 602, FIG. 6A) were each 12 GHz. Therefore, the totalsymbol rate for the dual-band signal (e.g., dual-band signal 604, FIG.6B) may reach up to 24 GHz. Carried by 64-QAM format, the totaldownstream capacity of a dual-polarization dual-band coherent signal maythus reach up to 288 Gb/s, which is considered at the present time tomore than adequately serve future subscriber needs within the coherentoptical access network paradigm. Also for the exemplary simulatedresults 600 illustrated in FIGS. 6A-D, the IF frequency used forup-conversion was set as 12 GHz, and the center frequencies of the RSB(S_(IF1)) and LSB (S_(IF2)) were correspondingly located at 12 GHz and−12 GHz, respectively. The electrical spectra of the single basebandsignal, dual-band signal after up-conversion, and down-converted signalafter digital filtering are shown in FIGS. 6A-C respectively. In thisexample, recovered constellation 608, FIG. 6D, represents a 64QAMconstellation of the recovered S₁ signal 606 under a SNR of 25 dB and alaser linewidth of 150 kHz.

According to results 600, the challenges presented by the recentadvances in COIL are successfully overcome within the coherent PONparadigm. The present systems and methods therefore improve uponCOIL-based architectural configurations for coherent PON P2MP networkssuch that residential home subscribers in particular are better able torealize performance levels comparable to present coherent P2P links, butat the cost of existing conventional direct detection PON systems.

Exemplary embodiments of optical communication systems and methods aredescribed above in detail. The systems and methods of this disclosurethough, are not limited to only the specific embodiments describedherein, but rather, the components and/or steps of their implementationmay be utilized independently and separately from other componentsand/or steps described herein. Additionally, the exemplary embodimentscan be implemented and utilized in connection with other access networksutilizing fiber and coaxial transmission at the end user stage.

As described above, the DOCSIS protocol may be substituted with, orfurther include protocols such as EPON, RFoG, GPON, Satellite InternetProtocol, without departing from the scope of the embodiments herein.The present embodiments are therefore particularly useful forcommunication systems implementing a DOCSIS protocol, and may beadvantageously configured for use in existing 4G and 5G networks, andalso for new radio and future generation network implementations.

Although specific features of various embodiments of the disclosure maybe shown in some drawings and not in others, such illustrativetechniques are for convenience only. In accordance with the principlesof the disclosure, a particular feature shown in a drawing may bereferenced and/or claimed in combination with features of the otherdrawings.

Some embodiments involve the use of one or more electronic or computingdevices. Such devices typically include a processor or controller, suchas a general purpose central processing unit (CPU), a graphicsprocessing unit (GPU), a microcontroller, a reduced instruction setcomputer (RISC) processor, an application specific integrated circuit(ASIC), a programmable logic circuit (PLC), a field programmable gatearray (FPGA), a digital signal processor (DSP) device, and/or any othercircuit or processor capable of executing the functions describedherein. The processes described herein may be encoded as executableinstructions embodied in a computer readable medium, including, withoutlimitation, a storage device and/or a memory device. Such instructions,when executed by a processor, cause the processor to perform at least aportion of the methods described herein. The above examples areexemplary only, and thus are not intended to limit in any way thedefinition and/or meaning of the term “processor.”

This written description uses examples to disclose the embodiments,including the best mode, and also enables a person skilled in the art topractice the embodiments, including the make and use of any devices orsystems and the performance of any incorporated methods. The patentablescope of the disclosure is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal language of the claims.

What is claimed is:
 1. A receiver for an optical communication network,comprising: an input portion operably coupled to an optical transportmedium of the optical communication network, the input portionconfigured to receive, from a remote parent laser source incommunication with the optical transport medium, a coherent dual-bandoptical signal including (i) a left-side band portion, (ii) a right-sideband portion, and (iii) a central optical carrier disposed within aguard band between the left-side band portion and the right-side bandportion; a first child laser; an optical circulator disposed between theinput portion and the first child laser, the optical circulatorconfigured to inject the received central optical carrier into the firstchild laser; an optical filter disposed between the optical circulatorand the first child laser, the optical filter configured to filter thecentral optical carrier from the coherent dual-band optical signal suchthat the first child laser is injection locked to a frequency of thecentral optical carrier; and a first downstream coherent opticalreceiving unit, wherein the first child laser is disposed between theoptical circulator and the first downstream coherent optical receivingunit.
 2. The receiver of claim 1, wherein the optical communicationnetwork includes a passive optical network (PON) system architecture. 3.The receiver of claim 1, further comprising at least one modem device.4. The receiver of claim 3, wherein the at least one modem comprises anoptical network unit (ONU).
 5. The receiver of claim 1, wherein the ONUfurther comprises a second child laser different from the first childlaser, wherein the second child laser is injection locked to thefrequency of the central optical carrier.
 6. The receiver of claim 5,wherein the ONU further comprises an uplink modulator, wherein theuplink modulator is configured to transmit a modulated, injection lockedupstream signal from one of the first and second child lasers to aremote transceiver associated with the remote parent laser source. 7.The receiver of claim 6, wherein the optical transport medium is asingle mode fiber further configured to carry both of the coherentdual-band optical signal and the modulated, injection locked upstreamsignal between the ONU and the remote transceiver.
 8. The receiver ofclaim 5, wherein the first and second child lasers each comprise aFabry-Perot laser diode (FPLD).
 9. The receiver of claim 8, wherein theFPLD of the first slave laser is configured to perform reamplificationand an erasing effect on the central optical carrier received from anoutput of the optical filter.
 10. The receiver of claim 9, wherein thefirst downstream coherent optical receiving unit is further configuredto utilize the central optical carrier, received from an output of theFPLD of the first slave laser, as a local oscillator (LO).
 11. Thereceiver of claim 10, wherein the first downstream coherent opticalreceiving unit further includes a digital signal processor (DSP)configured to down-convert the left-side band portion and the right-sideband portion onto a baseband signal.
 12. The receiver of claim 11,wherein the DSP is further configured to recover a filtered coherentdual-band optical signal from the down-converted left-side band portion,the down-converted right-side band portion, and the central opticalcarrier.
 13. The receiver of claim 1, wherein the optical filter is atleast one of a narrowband Bragg grating and a wave shaper.
 14. Thereceiver of claim 1, wherein the optical filter is at least one of ahighpass filter, a bandpass filter, and a lowpass filter.
 15. Thereceiver of claim 5, further comprising a polarization controllerdisposed between the optical filter and at least one of the first andsecond child lasers.
 16. The receiver of claim 1, wherein the coherentdual-band optical signal of the remote parent source includes acombination of two pairs intermediate frequencies (IFs), each pair ofIFs including at least one baseband in-phase (I) coherent signal and atleast one baseband quadrature (Q) coherent signal.
 17. The receiver ofclaim 16, wherein each of the baseband I/Q coherent signals aremodulated according to a quadrature amplitude modulation (QAM) format.18. The receiver of claim 1, wherein the central optical carrier of thecoherent dual-band signal is centered within a particular linewidth ofthe parent laser source.